1. Field of the Invention
The invention relates generally to measurement devices and more specifically to a device and method for determining the thickness of a layer of ice, or a layer containing a mixture of ice and water or other liquid.
2. Description of the Related Art
This applicant in U.S. Pat. No. 4,766,369, which is hereby incorporated by reference, disclosed a dual-geometry, capacitance-type ice thickness-measuring gauge. The gauge disclosed therein uses two capacitance sensors having greatly different electrode configurations and spacing to measure the thickness of surface ice independently of the temperature and impurities in the ice. That device works well if the ice is homogeneously solid, with no water or other liquid mixed in the ice, and when the temperature is not close to the melting point.
Ice formation often occurs with some liquid water present. In certain applications, e.g., aircraft icing, conditions, runway or roadway icing conditions, etc., it is critical to know that ice is forming and to determine its thickness even if it is not completely frozen. For such cases when liquid water is present, or when ice is not far from the freezing temperature, the electrical conductivity of the ice can be a significant or even dominant part of the total impedance across the ice. This conductivity, which is not accounted for in the dual geometry gage of U.S. Pat. No. 4,766,369, could result in indicated thickness measurements which are highly inaccurate if this term is not properly accounted for.